A signal encoded to an alternate mark inversion (AMI) code signal (hereinafter referred to as “AMI signal”) is a type of transmission signal used for transmission of digital signals. An AMI signal is used in a home bus system (HBS) and the like. An output AMI signal has three voltage levels: zero, positive, and negative, and the AMI signal flows through a positive signal line and a negative signal line. In a communication system using this signal, a logic “1” signal is set to the zero level; whereas a logic “0” signal is alternately set to positive and negative levels.
As disclosed in Japanese Patent Application Publication No. 8-195761, there is an air conditioner in which the exchange of data between an outdoor unit and an indoor unit is achieved by transmission of an AMI signal. Here, for example, the outdoor unit is equipped with a controller including a protocol controller, a transmitting circuit, a receiving circuit, and the like, and the transmitting circuit and the receiving circuit are connected to a positive signal line and a negative signal line that are extended to the indoor unit and the like. The receiving circuit receives an AMI signal from the positive signal line and the negative signal line; converts a positive pulse and a negative pulse of the signal to a low pulse (logic “0”), converts the zero level of the signal to a high pulse (logic “1”); and transmits the digital output to the protocol controller.
FIG. 9 shows an example of a receiving circuit used in a current air conditioner. This receiving circuit includes: coupling condensers C1, C2; an attenuator including resistors R4, R5, R7, and R8; a differential amplification circuit constituted by transistors Q1 and Q2 and constant current sources I1 and I2; resistors R1 and R2 for receiving a current output from the transistors Q1 and Q2 and converting it to a voltage; comparators U1 and U2 configured to judge an input signal by comparing each voltage output from the resistors R1 and R2 with a reference voltage V3; a gate circuit U3 configured to invert a logical sum of each output of the comparators U1 and U2; and the like. In FIG. 9, a symbol VCC5 represents a supply voltage. The receiving circuit receives an AMI signal from a positive signal line 91 and a negative signal line 92, and transmits a binary output signal from the gate circuit U3 to the protocol controller (not shown). In addition, in the receiving circuit shown in FIG. 9, a portion surrounded by dotted lines is embedded in an integrated circuit 59.